1. Field of the Invention
The present invention relates to a method of burn-in testing for a thermally-assisted head. Specifically, the present invention relates to a method of performing the burn-in test on laser diode units in a bar state which are used for a thermally-assisted head.
2. Description of the Related Art
In accordance with high recording density of a hard disk device (HDD), performance improvement in a thin film magnetic head is demanded. As the thin film magnetic head, a composite type thin film magnetic head is widely utilized that has a structure in which a reproducing head having a magneto resistance effect element (MR element) for reading and a reading head having an inductive electromagnetic transducer for writing are laminated.
A recording medium for magnetic recording is made from a discontinuous medium where magnetic micro particles are assembled, and each magnetic micro particle has a single magnetic domain structure. Since recording areas (each bit) are composed of a plurality of the magnetic micro particles, boundaries of the recording areas are uneven. In order to enhance the recording density, the unevenness of the boundaries of the recording area should be reduced. For that, it is effective to reduce the size of the magnetic micro particles; however, when the size of the magnetic micro particles is reduced, thermal stability is deteriorated in accordance with the decrease in volume of the magnetic micro particles. In order to enhance the thermal stability, it is preferred to utilize a magnetic material having a large magnetic anisotropy constant Ku; however, this leads to a difficulty in recording information with a conventional magnetic head because coercive force of the magnetic recording medium increases when anisotropy energy of the magnetic micro particles is increased. To solve this problem, a method of recording is proposed, in which heat as well as a magnetic field are applied at the time of recording so that coercive force is reduced. Such a method is referred to as thermally assisted magnetic recording. The thermally-assisted magnetic recording is similar to optical magnetic recording; however, the space resolution is realized by light in the optical magnetic recording. On the other hand, the space resolution is realized by a magnetic field in the thermally assisted magnetic recording.
As an example of the above-described thermally assisted magnetic recording, the specification of US2008/0002298 discloses a thermally assisted head including a surface-emitting diode.
In thermally-assisted recording technology, it is important to generate a minute light spot and also to determine an installation portion and a way to install a light source (laser diode unit). The specification of US 2008/0043360 discloses a structure of a head in which a laser diode unit incorporating a laser diode is attached to a back surface (an opposite surface of an air bearing surface) of a magnetic head slider in which a recording element and a reproducing element are mounted. The structure is referred to as a composite magnetic head slider structure.
The composite magnetic head slider structure has advantages over the prior art. (1) Since an integrated surface of a magnetic head slider is orthogonal to the air bearing surface, the composite magnetic head slider structure has an affinity for a head manufacturing process of a conventional hard disk device; (2) since a light source is positioned far away from the air bearing surface, a laser diode is less likely to receive a direct mechanical shock during operation of a hard disk device; and (3) since optical fiber and optical pick up lens are not utilized, cost is low and the number of manufacturing steps is small.
Also, the composite magnetic head slider structure has, in principle, a merit that characteristics of the laser diode and the magnetic head slider may be separately assessed. When the laser diode and the magnetic head slider are formed on one wafer in a wafer process and when either the laser diode or the magnetic head slider has a failure, its magnetic head should be treated as defective. Therefore, the yield rate may be decreased compared to the conventional magnetic head including only a magnetic head slider. On the other hand, with the composite magnetic head slider structure, it is possible to perform the testing on the laser diode by itself before the laser diode is mounted on the magnetic head slider. Therefore, since the defective laser diode may be eliminated and only a non-defective laser diode may be mounted on the magnetic head slider, the same yield rate as the conventional magnetic head may be maintained.
For the characteristic assessment of laser diode, a burn-in test is effective. The burn-in test measures and assesses variation over time under a high temperature situation and a current passage situation while a current passes through the laser diode.
When performing the characteristic assessment of the individual laser diode, the same as in the case of the head, it is preferable to perform the characteristic assessment and a reliability assessment by a unit of bar, namely in a state where laser diodes are aligned in a row. The laser diode is manufactured in the wafer process as in the magnetic head slider. Therefore, it is possible to manufacture a bar by cutting a wafer out. It requires a few hours to a few dozen hours to assess one laser diode with the burn-in test. Accordingly, performing the characteristic assessments of a large number of laser diodes at one time allows reduction of assessment cost, handling steps and time. When a length of a bar is approximately 80 mm, which is the length used in a manufacturing process of a conventional head, it is possible to dispose 100-200 pieces of laser diode on one bar.
FIG. 1A is a conceptual view illustrating characteristic assessment of laser diodes. The rightmost section of a bar 45 of submounts 133 illustrates a state where a laser diode 32 is separated from the submount 133. A laser diode unit 31 includes the submount 133 and the laser diode 32 mounted on the submount 133. The submounts 133 in the bar 45 state are held by a metallic fixture 53. The laser diode 32 includes electrodes 32a and 32b formed on a surface facing the submount 133 and an opposite surface thereof. A pad 44 is disposed on a surface of the submount 133 that faces the electrode 32a of the laser diode, and the pad 44 is electrically connected with the laser diode 32. The pad 44 is electrically connected with a drawing pad 44a. When probes 158 contact the drawing pad 44a and the electrode 32b to supply a current to the laser diode 32, the laser diode 32 emits laser light. The characteristics of the laser diode 32 are assessed by measuring light intensity of the laser light.
A conventional metallic needle is used as the probe 158 that supplies a current to the laser diode. However, there are disadvantages that the metallic needle is expensive and gives excessive mechanical stress to the laser diode. Specifically, for the test with the unit of bar, it is required to attach hundreds of the probes 158 on one card that holds the probes. Therefore, not only is the price of the probe itself expensive, but also all probes must be replaced when even one piece has a contact failure, which is not economical.
As illustrated in FIG. 1B, it may be considered to use cheap sheet probes 158′, which are used in a working process of the head. When performing the burn-in test using the sheet probes 158′, the sheet probes 158′ are disposed to be inclined at an angle range from 30 degrees to 60 degrees with respect to the drawing pads 44a and the electrodes 32b to contact the drawing pads 44a and the electrodes 32b. The sheet probes 158′ in which a large number of probes are integrated and each of the probes can move freely are common. Half of the probes 158′ are pressed to the drawing pads 44a, and another half of the probes 158′ are pressed to the electrodes 32b. 
The purposes of the contact by the probes 158′ is to provide an electrical contact and simultaneously to provide a preferable heat dissipation by contacting the bar 45 of the submounts 133 mounting the laser diodes 32 strongly with the fixture 53 supporting the bar 45. Therefore, the probes 158′ preferably provide a pressing load of approximately 0.02 N (2 gf) or more, which is relatively high, to the laser diode unit 31; but, on the other hand, the laser diode 32 may be possibly damaged when the excessive pressing load is provided. Therefore, an upper limit of the pressing load of the probe 158′ is desirably approximately 0.10N (10 gf).
The drawing pad 44a and the electrode 32b on the opposite surface of the laser diode have a height difference therebetween, which is the same thickness as a thickness D of the laser diode 32. Therefore, there is a large difference of the deformation pattern between the probe 158′ pressed to the drawing pad 44a and the probe 158′ pressed to the electrode 32b on the opposite surface of the laser diode 32. On the other hand, since the thickness D of each lot of the laser diodes 32 varies in a range of a few tens of microns, the deformation pattern of the probes 158′ also varies depending on the lot. Therefore, the following drawbacks occur.
For example, a case is considered in which the probe 158′ contacts the electrode 32b on the opposite surface of the laser diode to give a proper pressing load. In this case, damage that the laser diode 32 receives may be possibly suppressed. However, when a lot is used whose thickness D of the laser diode 32 is thinner, the pressing amount at the drawing pad 44a is relatively increased so that a pressing load is increased. As a result, the probe 158′ further deforms and slides on a surface of the drawing pad 44a, and thereby a contact point with the drawing pad 44a moves toward a front side of the sheet (illustrated by an arrow A). At worst, the probe 158′ tears loose from the drawing pad 44a, and it becomes difficult to perform the burn-in test in which the sheet probe 158′ stably contacts the drawing pad 44a. As described above, when the thickness of the laser diode 32 varies, it may become difficult for both the drawing pad 44a and the electrode 32b on the opposite surface of the laser diode to obtain the proper pressing load.
Such a drawback may be prevented, for example, when an extension part 44b is disposed with the drawing pad 44a and a length of the drawing pad 44a in a direction that the probe slides is extended. However, the size of the submount used for the thermally-assisted head is required to be small to the extent possible from a standpoint of flying stability of the head. Furthermore, since it needs to locate address information, etc. on the submounts, it is extremely difficult to dispose a drawing pad with a large area.
It is an object of the present invention to provide a method for performing a test on a plurality of laser diode units, each of in which a laser diode is mounted on a submount, in a bar state while solidly contacting a probe with a pad. Similarly, it is an object of the present invention to provide a bar of submounts that may be preferably used for the test.